1. Technical Field of the Invention
This invention relates to chemical vapor deposition of polysilicon directly onto the walls of a reaction chamber. More particularly, it relates to broad surface area chemical vapor deposition where a thin wall casing is used to construct the reaction chamber, and becomes the broad surface area form upon which the polysilicon deposit is made.
2. Background Art
One of the widely practiced conventional methods of polysilicon production is by depositing polysilicon in a chemical vapor deposition (CVD) reactor, and is referred to as Siemens method. In this method, polysilicon is deposited in a CVD reactor on high-purity thin silicon rods called "slim rods". Because of the high purity silicon from which these slim rods are fabricated, the corresponding electrical resistance of the slim rods is extremely high. Thus it is extremely difficult to heat this silicon "filament" using electric current, during the startup phase of the process.
Sometimes the slim rods are replaced by metallic rods that are more conductive and easier to heat with electrical current. This method is referred to as Rogers Heitz method. However, the introduction of metal into the chemical vapor deposition process introduces metal contamination. This contamination of the polysilicon yield is not acceptable in the semiconductor/microelectronics industry.
In the Siemens method, external heaters are used to raise the temperature of these high purity rods to approximately 400.degree. C. (centigrade) in order to reduce their electrical resisitivity. Sometimes external heating is applied in form of halogen heating or plasma discharge heating. However in a typical method, to accelerate the heating process, a very high voltage, in the order of thousands of volts, is applied to the rods. Under the very high voltage, a small current starts to flow in the slim rods. This initial flowing current generates heat in the slim rods, reducing the electrical resistance of the rods and permitting yet higher current flow and more heat.
This process of sending low current at high voltage continues until the temperature of slim rods reaches about 800.degree. C. At this temperature, the resistance of the high purity silicon rods falls very drastically and the high voltage source is switched to a low voltage source that is capable of supplying high current.
Referring to prior art FIG. 1, a CVD reactor consists of a base plate 23, quartz bell jar 17, chamber cover 24, bell jar supports 16, and heater 18 between the bell jar and the chamber cover. There is incorporated in base plate 23, a gas inlet 20 and a gas outlet 21, and electrical feedthroughs 19. A viewing port 22 provides for visual inspection of the interior.
In the prior art polysilicon manufacturing process by CVD, the silicon slim rod structure is assembled in the form of a hair pin by having a cross rod 2 placed horizontally on two long, spaced apart, vertical rods 1 and 3. The structure is mounted and connected so as to provide a current path between electrical feedthroughs 19. During the CVD process, polysilicon deposit accumulates uniformly on the slim rods; the deposit being shown here partially removed to show the slim rod structure.
Different users employ different methods for joining the horizontal rod to the vertical rods. One method requires a groove or a key slot at the top of each vertical rod. A small counter bore or conforming figment is formed on the ends of the horizontal rod so that it can be press fitted into the grooves to bridge the two vertical rods.
A typical prior art reactor consists of a complex array of subsystems. Two power sources are required, one power supply that can provide very high voltages and low current; and a second power supply that can sustain a very high current at relatively lower voltage. Also needed are the slim rod heaters and their corresponding power supply for preheating the slim rods. Another component is the high voltage switch gear. Moreover, the entire startup process is cumbersome and time consuming. Since the current drawn by the slim rods at around 800.degree. C. is of a run away nature, the switching of the high voltage to low voltage needs to be done with extreme care and caution.
Also, through this electric current method for heating the slim rods, the rods become an interior heat source losing tremendous amounts of heat via radiation to the surroundings. There is significant energy loss inherent in the existing practice.